1. Field of the Invention
The present invention generally relates to mixers. More specifically, the present invention provides a mixer using multiple phases of a local oscillator (LO) to provide improved high-order harmonic rejection.
2. Background Art
A mixer is a circuit that functions as a mathematical multiplier of two input signals. A mixer is used in a receiver to down-convert a radio frequency (RF) signal to a baseband signal or intermediate frequency (IF) signal. A mixer is used in a transmitter to up-convert a baseband signal or an IF signal to an RF signal. In this way, a mixer provides an output signal that is a frequency translated version of an input signal.
A mixer typically uses a local oscillator (LO) signal generated by a voltage controlled oscillator (VCO) to down-convert or up-convert an input signal. The local oscillator signal often approximates a square waveform, especially at low frequencies. Unlike a single-tone sinusoidal waveform, a square waveform contains harmonics at odd multiples of the fundamental frequency of the LO signal. Consequently, the output signal generated by a mixer using a square waveform as an LO signal can contain harmonics related to the LO frequency. Specifically, during up-conversion, the harmonics of the LO signal can produce replicas of the baseband or IF signal at odd multiples of the LO signal frequency. Further, during down-conversion, the harmonics of the LO signal can cause RF input signals at odd multiples of the LO frequency to directly interfere with each other in a resulting baseband or IF signal.
Using a sinusoidal waveform as an LO signal does not remove the harmonics from the output signal of the mixer. Specifically, due to the switching action of the transistors used to implement the mixer, harmonics related to the LO signal can still appear in the output signal. To remove the harmonics, pre-filtering is preformed when the mixer is used for down-conversion and post-filtering is preformed when the mixer is used for up-conversion.